The head of the European police agency Europol, Rob Wainwright, right, answers questions during an interview in The Hague, Netherlands, Friday, Jan. 16, 2015. French and German authorities arrested at least 12 people Friday suspected of links to the Islamic State group and a Paris train station was evacuated, with Europe on alert for new potential terrorist attacks. The police raids came the morning after Belgian authorities moved swiftly to pre-empt what they called a major impending attack, killing two suspects in a firefight and arresting a third in a vast anti-terrorism sweep that stretched into the night. (AP Photo/Peter Dejong)
(The Associated Press)

The head of the European police agency, Europol, Rob Wainwright, listens to questions during an interview in The Hague, Netherlands, Friday, Jan. 16, 2015. French and German authorities arrested at least 12 people Friday suspected of links to the Islamic State group and a Paris train station was evacuated, with Europe on alert for new potential terrorist attacks. The police raids came the morning after Belgian authorities moved swiftly to pre-empt what they called a major impending attack, killing two suspects in a firefight and arresting a third in a vast anti-terrorism sweep that stretched into the night. (AP Photo/Peter Dejong)
(The Associated Press)

Belgian police officers examine a wall in a street in Verviers, Belgium, Friday, Jan. 16, 2015. The street was blocked after security forces took part in anti-terrorist raids in Verviers, eastern Belgium on Thursday in which two people were killed and one arrested during a shootout in an anti-terrorist operation in the eastern city of Verviers. (AP Photo/Frank Augstein)
(The Associated Press)

THE HAGUE, Netherlands – The head of European Union police agency Europol says the large number of radicalized Muslim extremists across Europe, their lack of command structure and growing sophistication make it "extremely difficult" for law enforcement agencies to foil every terror attack.

Europol chief Rob Wainwright told The Associated Press in an interview Friday that Europe needs to cooperate more closely to prevent attacks like last week's deadly rampage in Paris.

Wainwright says that "the scale of the problem, the diffuse nature of the network, the scale of the people involved makes this extremely difficult for even very well-functioning counterterrorist agencies such as we have in France to stop every attack."

Wainwright says at least 2,500 and possibly up to 5,000 suspects have traveled from Europe to conflicts in Syria and Iraq.